Process for separating isobutene from C4 streams

ABSTRACT

A catalyst system for use in a reaction-distillation column which is a cloth belt having a plurality of pockets therein containing acid cation exchange resin arranged and supported by wire mesh intimately associated with the cloth pockets, particularly by coiling the cloth belt with the wire mesh disposed between the coils, i.e., in a spiral.

This is a continuation of application Ser. No. 928,397 filed July 27,1978, now U.S. Pat. No. 4,242,530.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the separation of isoolefins fromstreams containing mixtures of an isoolefin and the corresponding normalolefin. The present invention is especially useful for the separation ofisobutene from streams containing n-butenes.

2. Description of the Prior Art

Isoolefins of 4 carbon atoms are difficult to separate from thecorresponding normal olefin by simple fractionation because of thecloseness of their boiling points. In prior art processes as generallypracticed commercially, the isoolefin is selectively absorbed bysulfuric acid and the resulting isoolefin-containing sulfuric acidextract is then diluted and heated or treated with steam to separate theisoolefin.

Isobutene and diisobutene are of significant value having diverseapplications, for example, isobutene is one of the comonomers for butylrubber and diisobutene is an intermediate in the preparation ofdetergents. The n-butenes are required in pure form forhomopolymerization and as feeds for the oxidative production ofbutadiene. One manner of separating these components is to pass themixture through what is called a cold acid extraction procedure whereinthe stream is fed into a bath of concentrated sulfuric acid. Separationis achieved by virtue of the solubility of the isobutene in the sulfuricacid, the n-butenes and other hydrocarbons present passing overhead.

In an improved process reported in U.S. Pat. No. 3,531,539 to Tidwell,the C₄ feed stream containing isobutene was contacted with a molecularsieve at an elevated temperature and under sufficient pressure tomaintain a liquid phase, wherein the isobutene is converted todiisobutene which is easily separated from the stream by conventionalseparation techniques.

SUMMARY OF THE INVENTION

The present invention is a method for the separation of isoolefins,preferably having four to six carbon atoms, from streams containingmixtures thereof with the corresponding normal olefins. For example,streams containing isobutene and normal butene, isopentene and normalpentene and isohexene and normal hexene.

The method of the invention comprise (a) feeding a mixture containing anisoolefin and the corresponding normal olefin to a distillation columnreactor into a feed zone, (b) concurrently: (1) contacting said mixturewith a fixed bed acidic cation exchange resin, hereby catalyticallyreacting the isoolefin with itself to form a dimer, and (2)fractionating the resulting mixture of dimer and normal olefin, (c)withdrawing the dimer from the distillation column at a point below saidfeed zone and (d) withdrawing the normal olefin from the distillationcolumn reactor at a point above the feed zone, preferably above theacidic cation exchange resin.

A particular embodiment of the present invention is a method for theseparation of isobutene from a mixture comprising n-butene andisobutene. More generally, the invention is suitable for the separationof isobutene from a hydrocarbon stream which is substantially C₄hydrocarbons, such as n-butane, isobutene, n-butene, isobutane, andbutadiene (minor amounts of C₃ and C₅ hydrocarbons, i.e., less than 10%may be incidental components of such C₄ stream).

Briefly stated, the present method for separating isobutene comprises:

(a) feeding a mixture containing isobutene and n-butene to adistillation column reactor into a feed zone,

(b) concurrently:

(1) contacting said mixture with a fixed bed acidic cation exchangeresin, thereby catalytically reacting isobutene with itself to formdiisobutene, and

(2) fractionating the resulting mixture comprising diisobutene ann-butene,

(c) withdrawing said diisobutene from said distillation column reactorat a point below said feed zone and,

(d) withdrawing n-butene from said distillation column reactor at apoint above the feed zone.

The present invention also provides a new method for the preparation ofdimer, such as diisobutene.

The reaction system can be described as heterogeneous since the catalystremains as a distinct entity. The catalyst may be employed in suchconventional distillation packing shapes, as Raschig rings, Pall rings,saddles or the like. Similarly, the resin may be employed in a granularor bead form.

It has been found that the resin beads in a conventional fixed bed formtoo compact a mass for the upward flowing vapor and downward flowingliquid. However, it has been found that by placing the resin beads intoa plurality of pockets in a cloth belt, which is supported in thedistillation column reactor by open mesh knitted stainless steel wire bytwisting the two together, allows the requisite flows, prevents loss ofcatalyst, allows for the normal swelling of the beads and prevents thebreakage of the beads through mechanical attrition. This novel catalystarrangement is also part of the present invention.

The cloth may be of any material which is not attacked by thehydrocarbon feed or products under the conditions of the reaction.Cotton or linen are preferred. Briefly, the catalyst system comprises aplurality of closed cloth pockets arranged and supported in saiddistillation column reactor by wire mesh intimately associatedtherewith.

What readily distinguishes the present method from prior art is that theprior art has consistently employed a continuous liquid phase system forcontacting the isoolefin with the acidic catalyst, whereas the presentinvention carries out the method in a catalyst packed distillationcolumn which can be appreciated to contain a vapor phase and some liquidphase, as in any distillation. The dimerization reaction of isobuteneand the fractionation of the resultant n-butene-dimer mixture is carriedout simultaneously, i.e., concurrently. That is as the dimer is formedin the catalyst bed, the lower boiling n-butene is fractionated away inthe catalyst bed and removed overhead while the high boiling dimer dropsto the lower portion of the column.

The bulk type liquid phase reactions of the prior art had as one problemthe control of the temperature. The distillation avoids this problementirely.

DRAWINGS

FIG. 1 is a schematic representation of a catalytic distillation columnfor use in this invention.

FIG. 2 is an elevational view of a catalyst belt packing.

FIG. 3 is a cross sectional view of the catalyst belt packing takenalong 3--3 of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS

Mixed C₄ streams containing principally isobutane (I-C₄), normal butane(n-C₄), butene-1 (B-1), isobutene (I-B), trans butene-2 (TB-2) and cisbutene-2 (CB-2) (plus some minor impurities including butadiene), can betreated with cold sulfuric acid to remove isobutene and produce abutylene concentrate. The isobutene removed is recovered from the acidas a polymer (mostly dimer). The isobutene dimer (i.e., some otherisobutene polymers as well) and butene concentrate are valuableproducts.

A substitute route for accomplishing this same separation has beendiscovered. It has been found that a distillation column packed with aproperly supported acid catalyst can produce a bottom stream containingisobutene polymer (mostly dimer) and an overhead stream that isrelatively free of isobutene. This is surprising since the catalyst usedwould normally produce mostly heavy isobutene polymers and copolymers ifthe reaction were conducted in a conventional fixed bed.

The success of the catalytic distillation approach lies in anunderstanding of the principles associated with distillation. First,because the reaction is occurring concurrently with distillation, theinitial reaction product, diisobutene, is removed from the reaction zonenearly as quickly as it is formed. This removal of isobutene dimerminimizes further chaining to larger polymer lengths. Second, becauseall the C₄ components are boiling, the temperature of the reaction iscontrolled by the boiling point of the C₄ mixture at the systempressure. The heat of reaction simply creates more boil up, but noincrease in temperature. Third, the reaction has an increased drivingforce because the reaction products have been removed and can notcontribute to a reverse reaction (LeChatelier's Principle).

As a result, a great deal of control over the rate of reaction anddistribution of products can be achieved by regulating the systempressure. Also, adjusting the throughput (residence time=liquid hourlyspace velocity⁻¹) gives further control of product distribution anddegree of isobutene removal.

The temperature in the reactor is determined by the boiling point of theC₄ at any given pressure, that is, at constant pressure a change in thetemperature of the system, indicates a change in the composition in thecolumn. Thus, to change the temperature the pressure is changed. Byincreasing the pressure, the temperature in the system is increased.Generally, pressures in the range of 0 to 400 psig are or may beemployed, preferably 30 to 150 psig. For the C₄ stream, the presentreaction will be carried out generally at pressures in the range of 10to 300 psig, which will generally mean temperatures in the range of 10°to 100° C.

The reaction of isobutene with itself is equilibrium limited; however,by carrying out the reaction in a distillation column reactor andfractionating the formed product (diisobutene) downward away from thereaction zone, the equilibrium is constantly disrupted and hence thereaction never comes to equilibrium. This has the advantage of course,of achieving an effective 100% conversion (provided the catalyst bed isof sufficient length such that none of the isobutene escapes therefromto go overhead with the n-butenes). The adjustment of the size of thecatalyst bed is a mere mechanical step to be determined for each reactorand in accordance with the reaction conditions.

The system would normally be considered anhydrous; however, smallamounts of water often saturate the feed stream and represent about 400to 600 ppm thereof. The process will continue to operate in the samefashion, in the presence of this amount of water; however, the followingeffects have been observed: (1) all of the rates increase, however, thelower rates increase faster, (Although not mentioned above, those in theart will recognize that there is a reaction of isobutene with butene toproduce "codimer". This rate is normally much slower than thediisobutene rate). (2) the amount of codimer increases and (3) teritarybutanol is produced in small amounts.

The feed to the distillation column reactor is made at the lower end ofthe catalyst bed, preferably into the catalyst to allow immediatecontact of the isobutene with the catalyst.

A reflux is preferably included in the system. The reflux ratio couldvary over the range of 1 to 20:1. In practice, the higher ratio may beused to compensate for a short catalyst bed such as required forexperimental work. In commercial size unit the catalyst bed would beprovided so that lower reflux and hence higher unit productivity couldbe obtained.

The cation resins are those which have been used in the prior art forthis reaction. Catalysts suitable for the new process are cationexchangers, which contain sulfonic acid groups, and which have beenobtained by polymerization or copolymerization of aromatic vinylcompounds followed by sulfonation. Examples of aromatic vinyl compoundssuitable for preparing polymers or copolymers are: styrene, vinyltoluene, vinyl naphthalene, vinyl ethylbenzene, methyl styrene, vinylchlorobenzene and vinyl xylene. A large variety of methods may be usedfor preparing these polymers; for example, polymerization alone or inadmixture with other monovinyl compounds, or by crosslinking withpolyvinyl compounds; for example, with divinyl benzenes, divinyltoluenes, divinylphenylethers and others. The polymers may be preparedin the presence of absence of solvents of dispersing agents, and variouspolymerization initiators may be used, e.g., inorganic or organicperoxides, persulfates, etc.

The sulfonic acid group may be introduced into these vinyl aromaticpolymers by various known methods; for example, by sulfating thepolymers with concentrated sulfuric acid or chlorosulfonic acid, or bycopolymerizing aromatic compounds which contain sulfonic acid groups(see e.g., U.S. Pat. No. 2,366,007). Further sulfonic acid groups may beintroduced into these polymers which already contain sulfonic acidgroups; for example, by treatment with fuming sulfuric acid, i.e.,sulfuric acid which contains sulfur trioxide. The treatment with fumingsulfuric acid is preferably carried out at 0° to 150° C., and thesulfuric acid should contain sufficient sulfur trioxide after thereaction. The resulting products preferably contain an average of 1.3 to1.8 sulfonic acid groups per aromatic nucleus. Particularly, suitablepolymers which contain sulfonic acid groups are copolymers of aromaticmonovinyl compounds with aromatic polyvinyl compounds, particularly,divinyl compounds, in which the polyvinyl benzene content is preferably1 to 20% by weight of the copolymer (see, for example, German PatentSpecification No. 908,247).

The ion exchange resin is preferably used in a granular size of about0.25 to 1 mm, although particles from 0.15 mm up to about 1 mm may beemployed. The finer catalysts provide high surface area, but also resultin high pressure drops through the reactor. The macroreticular form ofthese catalysts is preferred because of the much larger surface areaexposed and the limited swelling which all of these resins undergo in anon-aqueous hydrocarbon medium.

Similarly, other acid resins are suitable, such as perflurosulfonic acidresins which are copolymers of sulfonyl fluorovinyl ethyl andfluorocarbon and described in greater detail in DuPont "Innovation",Volume 4, No. 3, Spring 1973 or the modified forms thereof as describedin U.S. Pat. Nos. 3,784,399; 3,770,567 and 3,849,243.

FIG. 1 illustrates schematically a typical distillation column in whichthe present process may be carried out. The column 10 was a one inchdiameter, five foot tall tube containing two feet of conventional glass1/16 inch helices 18 and three feet of the catalytic packing as shown inFIGS. 2 and 3.

FIG. 2 shows a cloth belt 30 wrapped in open mesh knitted stainess steelwire 31. The cloth bag 30 is composed of a plurality of vertical pockets32 sewn into the bag as shown in FIG. 3. Each pocket 32 is filled withresin catalyst 33.

The wire mesh provides the support for the catalyst (belt) and providessome degree of vapor passage through the catalyst beads, which otherwiseform a very compact bed which has a high pressure drop. Thus, the downflowing liquid is in intimate contact with the rising vapors in thecolumn.

Preferably the catalyst system comprises a plurality of said pockets ina single cloth belt, said cloth belt being coiled into a spiral andhaving wire mesh disposed between the coils of said cloth belt spiral,in which said pockets may be substantially parallel to the axis of saidspiral, and elongated along the axis of said spiral.

In commercial-scale operations, it is contemplated, catalyst packingwould be made up of alternating layers of resin-filled cloth beltssimilar to the ones described above, and a spacing material which couldbe of any convenient, suitable substance, such as a corrugated wirescreen or wire cloth or a knitted wire mesh. The layers would bearranged vertically to provide vapor passages between the belts. Thecylindrical resin-filled pockets could be oriented either vertically orhorizontally. For simplicity of fabrication and for better distributionof vapor flow passages, a vertical orientation is preferred. The heightof a section of this packing could be of any convenient dimension, froma few inches to several feet. For ease of assembly and installation, thepacking would be made into sections of the desired shape and size, eachsection fastened together with circumferential bands or tie wiresdepending on its size and shape. A complete assembly in a column wouldconsist of several sections, arranged in layers, with the orientation ofthe resin-filled belts turned at right angles in successive layers toimprove liquid and vapor flow distribution.

Other configurations which may be useful but with certain draw backswould be cages of wire cloth to contain catalyst beads, immersed inliquid on a conventional sieve tray. Disadvantages would be therestriction of vapor flow by the close weave of the wire, which may becompensated by allowing the beads to move freely in the cage, therebycausing attrition. Similarly, suspension of the catalyst on a tray wouldpresent problems of attrition, maintaining suspension and preventingcatalyst from leaving the tray.

In the laboratory column the bags are made in the form of a cloth beltapproximately six inches wide with narrow pockets approximately 3/4 inchwide sewn across the belt. The pockets are spaced about 1/8 inch apart.These pockets are filled with the catalyst beads to form approximatelycylindrical containers, and the open ends are then sewn closed toconfine the beads. This belt is then twisted into a helical form to fitinside the one inch column. Twisted in with the belt is also a strip ofan open mesh knitted stainless steel wire, which serves to separate theresin filled cloth pockets and provide a passage for vapor flow.

In operation, the isobutene containing C₄ feed enters through line 11into the lower end of the catalytic zone 24 which contains the catalystbag belt 25 as described. The temperature and pressure in the column aresuch that the C₄ boils up in the column, however, as the isobutenecontacts the catalyst, dimer is formed, which being higher boiling thanthe C₄ stream, passes to the bottom of the reactor where it is removedvia line 20 with a portion being recovered through line 21 and anotherportion recycled into reboiler 19 through line 22 and hence back intothe bottom of the column 10 through line 23.

Meanwhile, the n-butenes pass upward through the catalyst zone 24 andout of the column 10 via line 12 to condenser 13 hence into accumulator15 via line 14. A portion is recovered as butene concentrate from line16 and a portion is returned as reflux through line 17 into column 10.

EXAMPLES

In the following examples, the feed rate of C₄ 's to the column isadjusted to maintain a bottoms temperature which would correspond to lowC₄ concentration. The catalyst employed was Amberlyst 15, manufacturedby Rohm and Haas, Philadelphia, Pa. The feed had the followingcomposition:

    ______________________________________                                        Component              mole %                                                 ______________________________________                                        isobutane              2.8                                                    n-butane               8.6                                                    butene-1               24.6                                                   isobutene              50.5                                                   trans-butene-2         10.4                                                   cis-butene-2           3.1                                                    butadiene              .5                                                     ratio butene-1/butene-2                                                                              1.8                                                    ______________________________________                                    

                  Example 1                                                       ______________________________________                                        System Pressure = 70 psig                                                     Catalytic zone temperature = 60° C.                                    Results:                                                                                       Mole % component at                                                           LHSV.sup.-1 (Hrs)*                                                            0.2     0.7                                                  ______________________________________                                        OVERHEAD:                                                                     Component                                                                     isobutane          5.8       4.6                                              n-butane           20.0      28.7                                             butene-1           31.6      15.6                                             isobutene          7.4       1.5                                              trans-butene-2     26.7      37.7                                             cis-butene-2       8.4       11.9                                             butadiene          .18       .03                                              ratio butene-1/butene-2                                                                          1.2       .3                                               Overhead take-off (ml/hr)                                                                        480       160                                              bottom temp °C.                                                                           133       171                                              bottom C.sub.4 's (mole %)                                                                       9.8       2.7                                              BOTTOM:**                                                                     component                                                                     tert-butyl alcohol .14       .04                                              diisobutene        61.6      51.5                                             codimer            18.0      31.3                                             triisobutene       16.8      12.9                                             heavies            3.5       4.2                                              ______________________________________                                         *LHSV.sup.-1 is calculated by dividing the overhead takeoff rate into the     volume of resin in the catalytic zone. (The zone contained 115 grams (190     ml) of catalyst).                                                             **C.sub.4 's are normalized out                                          

                  Example 2                                                       ______________________________________                                        System Pressure = 40 psig                                                     Catalytic zone temperature = 40° C.                                    Results:                                                                                       mole % component at                                                           LHSV.sup.-1 (Hrs)*                                                            0.7     1.4                                                  ______________________________________                                        OVERHEAD:                                                                     Component                                                                     isobutane          4.8       5.7                                              n-butane           15.1      16.0                                             butene-1           44.3      43.8                                             isobutene          12.1      9.3                                              trans-butene-2     18.6      20.0                                             cis-butene-2       4.8       5.1                                              butadiene          .21       .2                                               ratio butene-1/butene-2                                                                          1.9       1.7                                              Overhead take-off (ml/hr)                                                                        160       80                                               bottom temp °C.                                                                           142       159                                              bottom C.sub.4 's (mole %)                                                                       3.0       .4                                               BOTTOM:**                                                                     Component                                                                     tert-butyl alcohol .26       .11                                              diisobutene        75.0      74.5                                             codimer            10.2      12.6                                             triisobutene       11.6      10.2                                             heavies            2.9       2.6                                              ______________________________________                                         *LHSV.sup.-1 is calculated by dividing the overhead takeoff rate into the     volume of resin in the catalytic zone. (The zone contained 115 grams (190     ml) of the catalyst).                                                         **C.sub.4 's are normalized out.                                         

The Invention Claimed Is:
 1. A catalyst system for use in areaction-distillation column comprising a plurality of closed clothpockets containing acid cation exchange resin, arranged and supported bywire mesh intimately associated with said closed cloth pockets.
 2. Thecatalyst system according to claim 1 comprising a plurality of saidpockets in a single cloth belt, said cloth belt being coiled into aspiral and having wire mesh disposed between the coils of said clothbelt spiral.
 3. The catalyst system according to claim 2 wherein saidpockets are substantially parallel to the axis of said spiral.
 4. Thecatalyst system according to claim 3 wherein said pockets are elongatedalong the axis of said spiral.